A magnetically-based torque sensor arrangement for a disc-like structure is described in U.S. Pat. No. 4,697,460 (Sugiyama et al). This patent discloses an arrangement for torque measurement in an automobile transmission. An energiser coil/detection coil assembly is non-contactingly placed adjacent a disc in which torque stress occurs. The energizing coil is A.C. energised to establish an alternating magnetic flux in a flux path through the disc that is torque sensitive. The detection coil senses torque-dependent changes in the circulating magnetic flux.
Another magnetically-based torque sensor arrangement for a disc-like structure such as a sprocket wheel is disclosed in PCT application WO01/13082 published 22nd Feb., 2001. This application discloses structures in which a permanent or stored magnetisation is associated with the disc to emanate a torque-dependent magnetic field component. The annular magnetisations employed for this purpose are further discussed below with reference to FIGS. 1-5.
A contrast can be drawn between using the disc structure as a medium for the A.C. coupling of an excitation coil to a detector and the use of stored or permanent magnetisation which itself can be regarded more as a D.C. type of magnetisation. This may be referred to herein as D.C. magnetisation.
The invention has particular, though not exclusive application, to torque transmission arrangements which, in operation, transmit torque in a pulsating or cyclic manner such as is found in the transmission of torque from the pedal cranks of a pedal cycle to the chain-engaging teeth of the pedal sprocket wheel, usually referred to as the chain wheel. In this case the torque is transmitted between an inner hub portion of the chain wheel and an outer annular tooth-bearing portion. The drive provided by the leg motion of the cyclist tends to approach zero when the pedal cranks are vertical, assuming the force exerted by the driving foot is vertically down. Without going into details of cycling motion and mechanics, it has been found that the maximum torque is usually exerted on the descending pedal crank in an arc extending to each side of the horizontal, though the arc may tend to advance (ascend) depending on the manner in which the cycle is being ridden. The generation of torque by the cyclist is made more complex because the rotational mounting of the pedals itself provides another free axis. The rider can angle the pedals to obtain the maximum mechanical input. Additionally the position of the rider is relevant, e.g. as between normal upright riding and a racing position. This maximum torque phase occurs twice for each revolution of the pedal cranks, once as each crank reaches the maximum transmission arc. This may be referred to as the maximum torque phase.
Heretofore it has not been possible to reliably and economically ascertain the torque transmitted in a chain wheel while the cycle is being ridden in normal use, and particularly the torque exerted in the maximum torque phase. More particularly it is desirable to be able to make such a measurement under normal cycling use without inconveniencing the cyclist.
The practice of the present invention provides a magnetically-based, non-contacting form of torque sensor arrangement having a magnetised transducer region which provides a torque-dependent magnetic field component and a non-contacting magnetic sensor. The preferred practice provides for automatic refreshment or compensation of the torque-dependent magnetic field which is to be sensed. The invention may be implemented to provide rotational speed measurement as well as torque sensing. The speed measurement may be implemented of its own right. The invention has particular application to cranks that are manually driven by hand (arm) or foot (leg) in which the torque generated is generally not uniform throughout the rotation of the crank and may also be applied in engine driven systems where the torque applied to a rotary shaft is non-uniform. In particular the applied torque may be pulsating, varying between maximum and minimum torque phases: in the case of a pedal cycle there are two pulses for each revolution of the chain wheel. The invention will be further described in relation to pedal bicycle.
The need for a reliable measurement of torque in normal use has arisen as a means of gauging the effort expended by a cyclist in propelling the machine. It has been proposed to assist the rider of a pedal cycle by providing a motor drive by way of an electric motor powered by a battery carried by the cycle. The battery should be as compact and light as possible consistent with providing the drive energy required for reasonable periods. Raising battery capacity increases weight. To obtain the best use of battery life, it is desirable that the cycle should not be propelled solely by the battery energy for extended periods but that the battery energy should be called on to supplement the energy of the cyclist. The battery energy is only utilised provided the cyclist is providing at least a certain threshold propulsion energy. The energy supplied by the rider can be gauged by the torque exerted on the chain wheel driving the cycle chain. The present invention enables us to provide a solution to the torque measurement problem which can be then used in controlling the energisation of the electric motor.